With the growing need for getting various types of temperature-sensitive cargo (such as blood plasma for human use) to various points across the country and around the world in short periods of time, there is a growing need for refrigerated air cargo containers to transport these temperature-sensitive cargoes and maintain them within a desired temperature range during their freight time. However, shipment of these temperature-sensitive commodities have not been very popular with the airlines because there has not been a reliable and effective refrigerated air cargo container for shipment of the goods. As a result of this, aircraft salespersons have not been willing to pursue the business of shipping temperature-sensitive commodities because of the risk involved in having to pay high dollar claims in the event that the shipment is lost or damaged. The refrigerated cargo industry has encountered many problems in attempting to effect a reliable and effective refrigerated container for shipment of temperature-sensitive commodities.
One of the main problems confronting the refrigerated container industry has been the problem of maintaining a reasonably uniform temperature within the container during the freight period. While temperature stability is much more important for some commodities than others, ideally a refrigerated container should maintain its contents within a .+-.2.degree. F. temperature range and it is desirable that the container maintain temperatures within .+-.4.degree. F. These temperature ranges should be maintained for a minimum of 36 hours when the cargo is frozen to temperatures near 0.degree. F. prior to shipment and for a minimum of 72 hours when the cargo is near freezing, i.e. in the 32.degree.-40.degree. F. range. Temperature-sensitive cargoes are normally carried either at around 0.degree. F. (for frozen beef) or just above freezing in the 32.degree.-40.degree. F. temperature range for most fresh produce. Some cargoes such as human blood plasma are carried in the -20.degree. F. temperature range. Normally these temperature-sensitive cargoes are precooled to their desired temperature before they are loaded into the refrigerated container; however, many cargoes are living organisms which continue to respire and generate heat while in transit.
Another problem facing the refrigerated cargo industry is that of maximizing the amount of interior space in the container which is useful or can be used by the shipper for storing his cargo. That is, a shipper is most interested in maximizing the amount of space which he has available to him for storing his cargo, since one extra cubic foot of interior space can save a shipper from $180-$590 per year in freight charges and rental of his cargo container. Therefore, it is desirable that most of the interior of the container be available for transporting cargo rather than occupied by the refrigeration mechanism or means for cooling the cargo.
Still another problem confronting the refrigeration container industry is that of minimizing the overall weight of the container and the refrigeration means. This weight, here defined as the tare weight (container weight.+-.weight of refrigerating mechanism.+-.weight of coolant) should be maintained at a very minimum, since an extra one pound in tare weight can cost an airliner between $0.70-$1.30 in fuel costs per year (based on about 50 flights per year). Also, if the commodity is very dense so that the container shipment is weight limited, an extra one pound of tare weight could cost $25.00 in lost freight income.
Perhaps the most important problem confronting the refrigeration cargo industry has been that of effecting a reliable container. This is a most important factor since if a refrigerated container fails to transport the cargo within the desired temperature range, then the entire shipment of cargo may well be lost. Such a loss can be rather costly to both the shipper and the airline in settling claims since one load of cargo weighing around 2,800 pounds could range from a value of $1,500.00 where fresh produce is being shipped to a value of $65,000.00 where human blood plasma is the commodity. In attempting to develop a reliable refrigerated cargo container, it is desirable to have a container where there are no moving parts or any otherwise mechanical devices, since all mechanical devices can and do break down at some time so that a passive system for the refrigerating cargo container is strongly preferred over some mechanical device.
With the need to develop a reliable refrigerated cargo container, there is also the need to develop a container which is safe for both the aircraft as well as the shipment. This safety factor is very important to aircraft personnel and crews since most aircraft personnel are hyper-sensitive about transporting any cargo that may present a threat to the safety of the aircraft or other cargoes being shipped. Because of certain aircraft regulations, an air cargo container usually cannot be connected to or draw power from an airline electric system without affecting the certification of the entire aircraft. Also, an air cargo container cannot emit any radio signals which might affect the aircraft's navigation system, thereby making electric motors and switches undesirable as being a part of the cargo container since they can emit signals, particularly when they malfunction. Also, as an additional safety feature in transporting cargo in aircraft, the refrigeration system cannot contain any gases at high pressures because of the danger of an explosion. As an added safety feature, a refrigerated container should only release carbon dioxide very slowly since high concentrations of carbon dioxide vapor could injure other living creatures such as pets/animals, which may also be carried in the same cargo compartment of the aircraft.
Perhaps an equally important factor as the reliability of a refrigerated cargo container is the cost of the container. The cost of the refrigerated container has to be kept fairly low in order to make the shipment of temperature-sensitive commodities by aircraft desirable. Costs of the refrigerated cargo containers should be limited to about 2-3 times the cost of a dry cargo container of the same size in order for it to be economical for use in shipping a broad range of temperature-sensitive commodities.
Certain prior art designs of refrigerated cargo containers have been attempted which have made use of exotic coolants or other exotic refrigeration mechanisms which have all resulted in increased operating and maintenance costs. There have been at least ten prior art designs for refrigerated air cargo containers over the past decade, but only four or five of these designs have been produced in any mentionable quantity, with only two of these designs being in use today.
It therefore follows that there is a need in the refrigeration container industry for a reliable container which can transport temperature-sensitive commodities within a temperature range which fluctuates by only at most 8.degree. F. and usually within 4.degree. F. from their desired temperatures, where the container is both safe to the cargo and to the aircraft carrying it and is also economical to the shipper. It is further desirable that this refrigerated container be such as to maximize the internal volume for storing cargo.